Python’s unittest module is based on the XUnit framework design by Kent Beck and Erich Gamma. The same pattern is repeated in many other languages, including C, Perl, Java, and Smalltalk. The framework implemented by unittest supports fixtures, test suites, and a test runner to enable automated testing.

The traceback module works with the call stack to produce error messages. A traceback is a stack trace from the point of an exception handler down the call chain to the point where the exception was raised. Tracebacks also can be accessed from the current call stack up from the point of a call (and without the context of an error), which is useful for finding out the paths being followed into a function.

The json module provides an API similar to pickle for converting in-memory Python objects to a serialized representation known as JavaScript Object Notation (JSON). Unlike pickle, JSON has the benefit of having implementations in many languages (especially JavaScript). It is most widely used for communicating between the web server and client in REST API, but is also useful for other inter-application communication needs.

The timeit module provides a simple interface for determining the execution time of small bits of Python code. It uses a platform-specific time function to provide the most accurate time calculation possible and reduces the impact of start-up or shutdown costs on the time calculation by executing the code repeatedly.

The trace module is useful for understanding the way a program runs. It watches the statements executed, produces coverage reports, and helps investigate the relationships between functions that call each other.

The pydoc module imports a Python module and uses the contents to generate help text at runtime. The output includes docstrings for any objects that have them, and all of the classes, methods, and functions of the module are described.

The multiprocessing module includes an API for dividing work up between multiple processes based on the API for threading . In some cases multiprocessing is a drop-in replacement, and can be used instead of threading to take advantage of multiple CPU cores to avoid computational bottlenecks associated with Python’s global interpreter lock.

Signals are an operating system feature that provide a means of notifying a program of an event, and having it handled asynchronously. They can be generated by the system itself, or sent from one process to another. Since signals interrupt the regular flow of the program, it is possible that some operations (especially I/O) may produce errors if a signal is received in the middle.